When a selectively permeable membrane, such as a reverse osmosis membrane, ultrafiltration membrane or the like, is used for, for example, concentration of a solution, desalting, production of pure water and other water treatment, the selectively permeable membrane will be contaminated by various pollutant substances, whereby it suffers from a decrease in the flux to cause a reduction in the selective permeability. Therefore, it has been practiced to wash such a selectively permeable membrane deteriorated in its performance using a detergent in order to recover the performance.
Hitherto, it has often been practiced to use an acid, alkali, organic solvent or surfactant as a detergent for washing a selectively permeable membrane deteriorated in its performance, such as flux. For example, Japanese Patent Kokai Sho-54-99783 A discloses a washing technique using an alkali as the detergent, by which it is difficult, due to severe contamination, to recover the flux within a brief time and the degree of flux recovery will never reach to 100%.
In Japanese Patent Kokais Sho-52-125475 A and Sho-58-119304 A and in others, a technique is disclosed in which an organic solvent is used at higher concentrations. However, this technique involves a danger of causing deterioration of the membrane or the membrane module, though a high washing effect may be attained for the case where the pollutant is an organic substance, so that the present situation is in that this technique is not applicable to practical apparatuses. For example, an organic solvent, such as methanol, ethanol or acetone, has a high washing ability for a reverse osmosis membrane or for a nano-filtration membrane, however, it causes, at higher concentrations, deterioration of the membrane or the membrane module. There is a problem that a high detergent effect is not attainable at a concentration not causing deterioration of the membrane or the membrane module.
In Japanese Patent Kokai Sho-55-51406 A, a washing technique using alkyl ethers of ethylene glycol is disclosed. However, alkyl ethers of ethylene glycol have both hydrophilic and hydrophobic groups and exhibit molecular structures close to monohydric alcohol. They are highly detrimental and emit a strong irritating smell and, hence, they are designated in the Law of PRTR (Environmental Pollution Release and Transfer Registration) and prescribed for their work environment assessment level to be not higher than 5 ppm, whereby there is a problem that a high application concentration is not allowed.
The object of the present invention is to propose a detergent for washing a selectively permeable membrane and a method for washing, with which it is able to realize a high detergent effect for a selectively permeable membrane and to cause no deterioration of the selectively permeable membrane and which are safe, not only for humans but also for the environment with superior handling easiness, and can wash a selectively permeable membrane having a deteriorated performance, such as the flux and so on, efficiently within a brief time to enable restoration of the performance.